Certain types of glasses and glass-ceramics can be chemically strengthened by an ion exchange process that may change the surface refractive index of the material. The strengthening is due to the formation of a near-surface compression layer that usually creates birefringence. The birefringence in turn corresponds to a change in refractive index profile in the glass.
There is increasing commercial interest in chemically strengthened glasses with anti-microbial (AM) surface properties. Such glasses can be fabricated using a double ion-exchange process (DIOX) wherein first IOX is performed for strengthening wherein a larger alkali ion such as K+ exchanges for a smaller alkali ion such as Na+ or Li+ in the original base glass. This gives a refractive-index profile similar to that shown in the plot of refractive index n vs. depth x shown in FIG. 1A. The first IOX is followed by a second IOX using an anti-microbial element, such as Ag+. This gives rise to a refractive index profile similar to that shown in FIG. 1B, wherein the profile contains a region R1 that has a relatively steep slope and is relatively shallow (e.g., just a few microns in depth x), along with a deeper region R2 of less slope but greater depth. In FIGS. 1A and 1B, the prism refractive index is denoted np, the interfacing fluid (which may also be referred as oil, immersion fluid or index-matching fluid) is denoted nf, the base substrate refractive index is denoted ns, and the elevated substrate surface refractive index is n0.
With the increased use of chemically strengthened glasses in such products as smart phones, computer screens and flat-panel televisions, there is an increasing need for nondestructive, high-throughput measurements of the surface stress in such glasses for quality control during manufacturing.
Unfortunately, conventional stress profile measurement techniques that employ prism coupling that work for the profile associated with FIG. 1A are inadequate for characterizing the stress profile associated with FIG. 1B because they cannot resolve the TM and TE mode spectra with adequate contrast.